The present invention relates to a distance measurement system for use with cameras or other optical devices.
As conventional distance measurement systems which determine the distance from an object to a camera or the like by detecting light reflected from the object by using photoelectric elements, a space frequency detection system (contrast detection system), a double image overlapping system (trigonometrical measurement system) and a time-lag measurement system are known. In the systems of the space frequency detection type, those that detect an extreme value of space frequency are in general use. In this type of space frequency detection system, a dip effect of a photoelectric element, that is, the phenomenon that the electrical resistance of the photoelectric element exhibits an the extreme value when the image of the object is in focus, is utilized. In the double image overlapping system, the image of the object is detected by a pair of photoelectric elements and the distance to the object is measured by changing the viewing angle of one photoelectric element while performing mirror-scanning to detect the point at which the two images completely overlap. This system is disclosed in Japanese Patent Publication No. 46-028,500 and U.S. Pat. No. 3,274,914. Honeywell type distance measurement system belongs to that type. A representative example of the time-lag measurement system, which measures the distance between the object and a predetermined point by measuring the time required for ultrasonic sound to travel from that point to the object and to return from the object to the point, is disclosed in Japanese patent publication No. 47-048,408.
The photoelectric elements for space frequency detection system are expensive, and that system involves a complicated signal processing procedure. The double image overlapping system requires some mechanical elements for mirror-scanning and has a limitation to the space obtained for the ground line. Moreover, the time-lag measurement system requires a ultrasonic sound generator which is rather large in size and expensive.